Method of constructing a microwave antenna

ABSTRACT

A method, simulation, and apparatus are provided that are highly suitable for treatment of benign prostatic hyperplasia (BPH). A catheter is disclosed that includes a small diameter disk loaded monopole antenna surrounded by fusion material having a high heat of fusion and a melting point preferably at or near body temperature. Microwaves from the antenna heat prostatic tissue to promote necrosing of the prostatic tissue that relieves the pressure of the prostatic tissue against the urethra as the body reabsorbs the necrosed or dead tissue. The fusion material keeps the urethra cool by means of the heat of fusion of the fusion material. This prevents damage to the urethra while the prostatic tissue is necrosed. A computer simulation is provided that can be used to predict the resulting temperature profile produced in the prostatic tissue. By changing the various control features of the catheter and method of applying microwave energy a temperature profile can be predicted and produced that is similar to the temperature profile desired for the particular patient.

This application is a division of application Ser. No. 09/511,961, filedFeb. 23, 2000 This application is a continuation-in-part of Ser. No.08/641,045 U.S. Pat. No. 5,904,709, filed Apr. 17, 1996 and issued May18, 1999, and is a continuation-in-part of U.S. patent application Ser.Nos. 09/162,457 now abandoned and 09/154,622 each filed Sep. 16, 1998,now U.S. Pat. No. 6,175,768 and is a continuation-in-part of U.S.Application Ser. No. 09/154,989 filed Sep. 17, 1998 now U.S. Pat. No.6,134,476.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus, methods, and computersimulations highly suitable for treatment of benign prostatichyperplasia (BPH) and, more particularly, to a unique catheter formicrowave treatment of BPH to necrose prostatic tissue while protectingurethral tissue and computer simulations relating to the same.

2. Description of Prior Art

Benign prostatic hypertrophy or hyperplasia (BPH) is one of the mostcommon medical problems experienced by men over 50 years old. Urinarytract obstruction due to prostatic hyperplasia has been recognized sincethe earliest days of medicine. Hyperplastic enlargement of the prostategland, or enlargement due to abnormal but benign multiplication of thecells thereof, often leads to compression of the urethra therebyresulting in obstruction of the urinary tract. Common symptoms thatdevelop from this condition may include more frequent urination,decrease in urinary flow, nocturia, pain, and discomfort. The incidenceof BPH in men over 50 years of age is approximately 50 percent andincreases to over 75 percent in men over 80 years of age. Symptoms ofurinary obstruction occur most frequently between the ages of 65 and 70when approximately 65 percent of men in the age group have prostaticenlargement.

When treatment by drug therapy is not sufficiently effective, surgicalprocedures for treating BPH are available but have potential sideeffects. General surgical risks apply such as anesthesia relatedmorbidity, hemorrhage, coagulopathies, pulmonary emboli, electrolyteimbalance, and the like. Other problems that may occur from surgicalcorrection include cardiac complications, bladder perforation,incontinence, infection, urethral or bladder neck stricture, retentionof prostatic chips, and infertility. Due to the problems of surgery,many or even most patients delay treatment. However, the delay oftreatment may lead to other complications including obstructive lesionin the prostate, chronic infection, and the like. Therefore it isunquestionable that a need exists for improved surgical or non-surgicalmethods for treating BPH.

Microwaves and other techniques have been used to necrose malignant,benign, and other types of cells and tissues including glandular andstromal nodules characteristic of benign prostate hyperplasia. However,problems encountered include a lack of focusing or direction of theenergy thereby resulting in damage of healthy tissue.

The following patents disclose attempts to solve the above discusseddifficult problems and related problems.

U.S. Pat. No. 5,904,709, issued May 18, 1999, to Arndt et al., andincorporated herein, discloses a method and apparatus for propagatingmicrowave energy into heart tissues to produce a desired temperatureprofile therein at tissue depths sufficient for thermally ablatingarrhythmogenic cardiac tissue to treat ventricular tachycardia and otherarrhythmias while preventing excessive heating of surrounding tissues,organs, and blood. A wide bandwidth double-disk antenna is effective forthis purpose over a bandwidth of about six gigahertz. A computersimulation provides initial screening capabilities for an antenna suchfrequency, power level, and power application duration. The simulationalso allows optimization of techniques for specific patients orconditions. In operation, microwave energy between about 1 Gigahertz and12 Gigahertz is applied to the monopole microwave radiator having asurface wave limiter. A test setup provides physical testing ofmicrowave radiators to determine the temperature profile created inactual heart tissue or ersatz heart tissue. Saline solution pumped overthe heart tissue with a peristaltic pump simulates blood flow. Opticaltemperature sensors disposed at various tissue depths within the hearttissue detect the temperature profile without creating anyelectromagnetic interference. The method may be used to produce adesired temperature profile in other body tissues reachable by cathetersuch as tumors and the like.

U.S. Pat. No. 5,843,144, issued Dec. 1, 1998, to Rudie et al., disclosesa method for treating an individual with diseased prostatic tissue, suchas benign prostatic hyperplasia, including inserting a catheter into aurethra to position a microwave antenna located within the catheteradjacent a prostatic region of the urethra. A microwave antenna is thendriven within a power range for applying microwave energy substantiallycontinuously to prostatic tissue to heat the prostatic tissuesurrounding the microwave antenna at a temperature and for a time periodsufficient to cause necrosis of the prostatic tissue.

U.S. Pat. No. 5,843,026, issued Dec. 1, 1998, to Edwards et al.,discloses a method and apparatus for delivering controlled heat toperform ablation to treat the benign prosthetic hypertrophy orhyperplasia (BPH). According to the method and the apparatus, the energyis transferred directly into the tissue mass which is to be treated insuch a manner as to provide tissue ablation without damage tosurrounding tissues. Automatic shut-off occurs when any one of a numberof surrounding areas to include the urethra or surrounding mass or theadjacent organs exceed predetermined safe temperature limits. Theconstant application of the radio frequency energy over a maintaineddetermined time provides a safe procedure which avoids electrosurgicaland other invasive operations while providing fast relief to BPH with ashort recovery time. The procedure may be accomplished in a doctor'soffice without the need for hospitalization or surgery.

U.S. Pat. No. 5,830,179, issued Nov. 3, 1998, to Mikus et al., disclosesa stent system and method for use in the prostate gland. The stent ismade of a shape memory alloy such as nitinol, and has a low temperaturemartensite state, with a martensite transition temperature below bodytemperature, and a high temperature austenite state, with an austenitetransition temperature at or above body temperature, and a memorizedshape in the high temperature austenite state which is a helical coil ofdiameter large enough to hold the prostatic urethra open. The stent isused to heat the prostate and is left in the prostatic urethra while theprostate heals. After the prostate is substantially healed, the stent iscooled to its martensite state and is easily removed from the urethra.

U.S. Pat. No. 5,800,486, issued Sep. 1, 1998, to Thome et al., disclosesan intraurethral catheter which includes a microwave antenna and acooling lumen structure substantially surrounding the antenna. A coolingballoon partially surrounds the cooling lumens on one side of thecatheter adjacent the microwave antenna. The cooling balloon improveswall contact between the catheter and a wall of the urethra to improvecooling of the urethra. The cooling balloon communicates with thecooling lumen structure to permit circulation of cooling fluid throughthe cooling balloon.

U.S. Pat. No. 5,800,378, issued Sep. 1, 1998, to Edwards et al.,discloses a medical probe device comprising a catheter having a styletguide housing with one or more stylet ports in a side wall thereof and astylet guide for directing a flexible stylet outward through the styletport and through intervening tissue at a preselected, adjustable angleto a target tissue. The total catheter assembly includes a stylet guidelumen communicating with the stylet port and a stylet positioned in saidstylet guide lumen for longitudinal movement from the port throughintervening tissue to target tissue. The stylet can be an electricalconductor enclosed within a non-conductive layer, the electricalconductor being a radio frequency electrode. Preferably, thenon-conductive layer is a sleeve which is axially moveable on theelectrical conductor to expose a selected portion of the electricalconductor surface in the target tissue. The stylet can also be amicrowave antenna. The catheter can include one or more inflatableballoons located adjacent to the stylet port for anchoring the catheteror dilation. Ultrasound transponders and temperature sensors can beattached to the probe end and/or stylet. The stylet guide can define astylet path from an axial orientation in the catheter through a curvedportion to a lateral orientation at the stylet port.

U.S. Pat. No. 5,755,754, issued May 26, 1998, to Rudie et al., disclosesan intraurethral, Foley-type catheter shaft containing a microwaveantenna capable of generating a cylindrically symmetrical thermalpattern, within which temperatures are capable of exceeding 45° C. Theantenna, which is positioned within the shaft, is surrounded by meanswithin the shaft for absorbing thermal energy conducted by the tissueand asymmetrically absorbing electromagnetic energy emitted by theantenna—a greater amount of electromagnetic energy being absorbed on oneside of the shaft. This asymmetrical absorption alters the thermalpattern generated by the microwave antenna, making it cylindricallyasymmetrical, which effectively focuses microwave thermal therapy towardundesirous benign tumorous tissue growth of a prostate anterior andlateral to the urethra, and away from healthy tissue posterior to theurethra.

U.S. Pat. No. 5,733,315, issued Mar. 31, 1998, to Burdette et al.,discloses an apparatus for applying thermal therapy to a prostate gland,comprising a support tube having a longitudinal central passageway, apower lead channeled through the longitudinal central passageway and anultrasound crystal disposed around at least part of the support tube.The ultrasound crystal is coupled to the power lead which provides thepower to energize the ultrasound crystal and generate ultrasound energyproviding thermal therapy to the prostate gland. The ultrasound crystalfurther includes inactivated portions for reducing ultrasound energydirected to the rectal wall of the patient. A sealant is disposed incontact with the ultrasound crystal allowing vibration necessary forefficient ultrasound energy radiation for the thermal therapy to theprostate gland.

U.S. Pat. No. 5,720,718, issued Feb. 24, 1998, to Rosen et al.,discloses a medical probe device comprising a catheter having a styletguide housing with at least one stylet port in a side thereof and styletguide means for directing a flexible stylet outward through at least onestylet port and through intervening tissue to targeted tissue. Thestylet comprises an electrical central conductor which is enclosedwithin an insulating or dielectric sleeve surrounded by a conductivelayer terminated by an antenna to selectively deliver microwave or radiofrequency energy to target tissue. One embodiment includes theelectrical conductor being enclosed within a non-conductive sleeve whichitself is enclosed within a conductive sleeve in a coaxial cablearrangement to form a microwave transmission line terminated by anantenna. Another embodiment includes a resistive element near the distalend of the stylet which couples the center electrode to an outerconductor to generate joulian heat as electromagnetic energy is applied,such as an RF signal.

U.S. Pat. No. 5,643,335, issued Jul. 1, 1997, to Reid et al., disclosesa system for treatment of benign prostatic hyperplasia withinintraprostatic tissue surrounding a urethra. The system includes anintraurethral catheter having an intraurethral catheter shaft. Anantenna is located within the catheter shaft for delivering heat to theintraprostatic tissue surrounding the urethra. Coolant fluid iscirculated through a chamber located between the catheter shaft and theurethral wall.

U.S. Pat. No. 5,620,480, issued Apr. 15, 1997, to Eric N. Rudie,discloses a method for treating an individual with benign prostatehyperplasia. The method includes inserting a catheter into a urethra soas to position an energy emitting element located within the catheteradjacent a prostatic region of the urethra. A fluid is circulated withinthe catheter until the fluid stabilizes at a prechilled temperature. Anenergy emitting element is then energized sufficient to heat prostatictissue surrounding the energy emitting element.

U.S. Pat. No. 5,599,294, issued Feb. 4, 1997, to Edwards et al.,discloses a medical probe device comprising a catheter having a styletguide housing with one or more stylet ports in a side wall thereof andguide means for directing a flexible stylet outward through the styletport and through intervening tissue at a preselected, adjustable angleto a target tissue. The stylet can be an electrical conductor enclosedwithin a non-conductive layer, the electrical conductor being a radiofrequency electrode. Preferably, the non-conductive layer is a sleevewhich is axially moveable on the electrical conductor to expose aselected portion of the electrical conductor surface in the targettissue. The stylet can also be a microwave antenna.

U.S. Pat. No. 5,575,811, issued Nov. 19, 1996, to Reid et al., disclosesa system for treatment of benign prostatic hyperplasia withinintraprostatic tissue surrounding a urethra. The system includes anintraurethral catheter having an intraurethral catheter shaft. Anantenna is located within the catheter shaft for delivering heat to theintraprostatic tissue surrounding the urethra. Coolant fluid iscirculated through a chamber located between the catheter shaft and theurethral wall.

U.S. Pat. No. 5,509,929, issued Apr. 23, 1996, to Hascoet et al.,discloses a urethral probe having a front part and a rear part, and amicrowave antenna connected to an external device for generatingmicrowaves. The microwave antenna has its primary active heating partarranged in the urethral probe to be directed onto the prostatic tissuelocated at least at the level of the bladder neck in the workingposition. The urethral probe constitutes an essential element of adevice for the therapeutic treatment of tissues by thermotherapy, moreparticularly tissues of the bladder of a human being.

U.S. Pat. No. 5,464,437, issued Nov. 7, 1995, to Reid et al., disclosesa system for treatment of benign prostatic hyperplasia withinintraprostatic tissue surrounding a urethra. The system includes anintraurethral catheter having an intraurethral catheter shaft. Anantenna is located within the catheter shaft for delivering heat to theintraprostatic tissue surrounding the urethra. Coolant fluid iscirculated through a chamber located between the catheter shaft and theurethral wall.

U.S. Pat. No. 5,413,588, issued May 9, 1995, to Rudie et al., disclosesan intraurethral, Foley-type catheter shaft containing a microwaveantenna capable of generating a cylindrically symmetrical thermalpattern, within which temperatures are capable of exceeding 45° C. Theantenna, which is positioned within the shaft, is surrounded by meanswithin the shaft for absorbing thermal energy conducted by the tissueand asymmetrically absorbing electromagnetic energy emitted by theantenna—a greater amount of electromagnetic energy being absorbed on oneside of the shaft. This asymmetrical absorption alters the thermalpattern generated by the microwave antenna, making it cylindricallyasymmetrical, which effectively focuses microwave thermal therapy towardundesirous benign tumorous tissue growth of a prostate anterior andlateral to the urethra, and away from healthy tissue posterior to theurethra.

U.S. Pat. No. 5,366,490, issued Nov. 22, 1994, to Edwards et al.,discloses a medical probe device comprising a catheter having a styletguide housing with one or more stylet ports in a side wall thereof andguide means for directing a flexible stylet outward through the styletport and through intervening tissue at a preselected, adjustable angleto a target tissue. The stylet can also be a microwave antenna.

U.S. Pat. No. 5,312,392, issued May 17, 1994, to Hofstetter et al.,discloses a method of treating benign prostatic hyperplasia employingthe steps of inserting a diffusing light guide into a prostrate lobe andproviding laser power to the diffusing light guide in order to necrosesurrounding tissue. The diffusing light guide can be inserted into thecentral or lateral prostrate lobes by inserting a needle and a trocartransperineally into the middle of the lateral lobe, removing thetrocar, inserting the diffusing light guide, and monitoring the positionof the needle, trocar, and diffusing light guide using ultrasound. Thediffusing light guide can also be inserted into the central or lateralprostrate lobes transurethrally and positioned with the aid of anurethroscope.

U.S. Pat. No. 4,967,765, issued Nov. 6, 1990, to Turner et al.,discloses a urethral inserted applicator for prostate hyperthermiaincluding a multi-tube, balloon type catheter. The catheter includesfirst and second closed end fluid dry tubes, respectively, for a helicalcoil antenna type applicator, and a microwave type temperature sensorfor measuring the temperature of the prostate tissue, and an open fluidreceiving tube. A microwave generator supplies electromagnetic energy tothe applicator. A comparator is connected to the temperature sensor anda temperature reference potentiometer for comparing the actual tissuetemperature level with a desired temperature level and outputtingcontrol signals to the microwave generator for controlling the output tothe applicator. The coil type applicator is an elongated coil having atip end connected to the center conductor of a coaxial cable and anopposite end connected to the outer conductor of the coaxial cable. Asheet or sheath of insulation material covers the coil antenna forinsulating the coil from the tissue and the thickness of the sheet maybe varied to provide uniform tissue heating along the length of thecoil. The balloon of the catheter engages the body's bladder to positionthe applicator properly during the treatment.

The above cited prior art does not provide an easily fabricated catheterthat may be fabricated with variations useful for individual patients, acomputer simulation to predict the effect of procedural techniques, anda relatively quick procedure that may be performed in minutes to necroseprostatic tissue while protecting healthy tissue. Consequently, there isa strong need for improved treatment techniques that accurately pinpointand necrose selected prostatic tissue while protecting the urethra andother healthy structures by cooling and by selectively directingmicrowave radiation. Those skilled in the art have long sought and willappreciate the present invention that addresses these and otherproblems.

SUMMARY OF THE INVENTION

The present invention provides a procedure, apparatus, and computersimulation for treating benign prostatic hyperplasia (BPH). The computersimulation may be used to predict a temperature profile that will beproduced given the various inputs related thereto. Alternatively, it maybe used to provide suitable procedure variables such as frequency, timeduration, and power level, given the desired temperature profile. Theprocedure has a treatment time of only a few minutes and is designed toprevent damage to healthy tissue such as the urethra. The antenna may bemade directional to protect structures such as the colon and structuressuch as ducts radially outside the urethra. For purposes of the presentinvention, a catheter is assumed to include a probe, cannula or othermedical device for insertion into the body such as into the urethra fortreatment purposes.

For this purpose, a transcatheter microwave antenna is disclosed thatcomprises a catheter preferably formed from a microwave transmissionline having first and second opposing ends. The first end may be adaptedfor connection to a microwave power source. The microwave transmissionline preferably has a center conductor and an outer conductor. Amicrowave antenna is disposed on the second end of the microwaveantenna. A layer of fusion material is disposed radially outward of themicrowave antenna. The fusion material is operable to be in a firstphysical state prior to operation of the microwave antenna. The fusionmaterial is alterable to a second physical state from the first physicalstate to provide heat of fusion cooling adjacent the catheter duringoperation of the microwave antenna. In a preferred embodiment, thefusion material may be in a solid physical state prior to operation ofthe microwave antenna and is operable for melting to a liquid stateduring operation of the microwave antenna so as to provide coolingradially outward from the catheter. The fusion material in the solidstate is substantially flexible. The fusion material is substantiallytransparent to microwave radiation so as to absorb little energydirectly from the microwave radiation. In one embodiment, the fusionmaterial has a melting point in the range of from approximately eightyto one hundred degrees Fahrenheit.

Electrical insulating material is preferably provided between the centerconductor and the outer conductor. The microwave antenna may be disposedwithin the electrical insulating material in a preferred embodiment. Anouter sheath preferably but not necessarily surrounds the fusionmaterial. The layer of fusion material is provided in surroundingrelationship to the microwave antenna. In one embodiment, the fusionmaterial is comprised of a crystalline material. In another embodiment,the fusion material is comprised of powdered material. The fusionmaterial, in one embodiment, may be comprised of either dibasic sodiumphosphate or phosphonium chloride.

Preferably a tubular conductor acts as an attenuator of microwaves andmay be mounted at or near a surface of the catheter and may be axiallypositioned on the catheter adjacent the microwave antenna. In oneembodiment of the invention, material is provided for absorbingmicrowave heat energy on one side of the microwave antenna so as to makethe microwave antenna directional.

A method of constructing a transcatheter microwave antenna may includeproviding a coaxial cable with a center conductor and an outerconductor. The coaxial cable is adapted for connection to a microwavepower source. A microwave antenna at one end of the coaxial cable and alayer of fusion material is provided adjacent the microwave antenna. Thefusion material is operable to be in a first physical state prior tooperation of the microwave antenna and may be alterable to a secondphysical state from the first physical state to provide heat of fusioncooling adjacent the catheter during operation of the microwave antenna.The fusion material is selected to have a melting temperature in therange from about eighty to one hundred degrees Fahrenheit.

A method for selective thermal necrosing of a tissue to be treated whilelimiting thermal damage to a protected tissue that comprises positioningan energy radiator adjacent to heat the tissue to be treated such thatthe protected tissue is between the tissue to be treated and the energyradiator. A layer of fusion material is positioned between the energyradiator and the protected tissue such that convection transfer ofenergy may occur between the protected tissue and the layer of fusionmaterial. Energy is radiated from the energy radiator to heat the tissueto be treated. The temperature rise in the protected tissue is limitedby convection transfer of energy between the protected tissue and thefusion material. The temperature rise in the fusion material is limitedby changing the fusion material from a first physical state to a secondphysical state due to the convection transfer of energy between theprotected tissue and the fusion material. The fusion material preferablyhas a melting point below body temperature. The energy radiator may becontrolled so as to direct energy from the energy radiator in one ormore selected directions toward the tissue to be treated. This may beaccomplished by positioning the energy absorbing material adjacent theenergy radiator to absorb energy from the energy radiator. A frequencyof operation may be selected based on a distance of the energy radiatorto the tissue to be treated so as to focus energy to the tissue to betreated.

Thus, in operation a method for selective thermal ablation of a tissueto be treated is provided that limits thermal damage to protectedtissues and comprises positioning fusion material adjacent the protectedtissue to permit convection transfer of energy between the fusionmaterial and the protected tissue, the fusion material preferably has amelting point lower than body temperature. Energy is radiated throughthe fusion material and through the protected tissue and into the tissueto be treated. Microwaves along the outside of the catheter areattenuated with a tubular conductor. The microwave antenna may beadjusted so that the radiation points deposit energy at a selecteddistance of the tissue to be treated from the microwave antenna.

A computer program is provided for simulating treatment in biologicaltissue which comprises providing at least one antenna characteristic forthe microwave antenna. At least one tissue characteristic is provided ofthe tissue into which microwave energy is to be deposited. At least onecooling characteristic of cooling substances through which microwavesare transmitted is provided. A frequency of operation and a power levelmay be provided. A delivery time for the microwave energy may beprovided. A temperature profile produced within the biological tissuemay be determined. The temperature versus distance radially outward fromthe microwave antenna may be displayed graphically in some manner. Thechanges in the temperature profile with time may be displayed. Thecomputer program may also include a characteristic of absorptionmaterial for absorbing the microwave energy in at least one direction soas to alter the temperature profile accordingly.

One object of the present invention is to provide an improved instrumentand method for necrosing certain tissue while protecting other tissue.

Another object of the present invention is to provide an improvedinstrument, method, and computer simulation for treating benignprostatic hyperplasia.

Yet another objective of the present invention is to provide a treatmentthat necroses prostatic tissue but protects other tissue such as theurethra.

Any listed objects, features, and advantages are not intended to limitthe invention or claims in any conceivable manner but are intendedmerely to be informative of some of the objects, features, andadvantages of the present invention. In fact, these and yet otherobjects, features, and advantages of the present invention will becomeapparent from the drawings, the descriptions given herein, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, in section, of a catheter with amicrowave antenna surrounded by a layer of fusion material that coolssurrounding tissue through heat of fusion;

FIG. 2 is a table showing computer simulation results with temperatureversus depth for a catheter in accord with the present invention;

FIG. 3 is a three dimensional depiction of an isothermal profileproduced with the catheter of FIG. 1 operating at 918 KHz for 120seconds and other selected system parameters;

FIG. 4 is a three dimensional depiction of an isothermal profileproduced with the catheter of FIG. 1 operating at 918 KHz for 180seconds and other selected system parameters;

FIG. 5 is a three dimensional depiction of an isothermal profileproduced with the catheter of FIG. 1 operating at 450 KHz for 120seconds and other selected system parameters;

FIG. 6 is a schematic representation of a microwave thermal simulationmodel for simulated in-vivo operation of the catheter of FIG. 1; and

FIG. 7 is an elevational view, in section, of a catheter with amicrowave antenna surrounded by a layer of material that coolssurrounding tissue through heat of fusion along with a energy absorptionlayer for absorbing microwave energy so that the microwave antenna isdirectional.

While the present invention will be described in connection withpresently preferred embodiments, it will be understood that it is notintended to limit the invention to those embodiments. On the contrary,it is intended to cover all alternatives, modifications, and equivalentsincluded within the spirit of the invention and as defined in theappended claims.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

The techniques, catheter, and computer simulation of the presentinvention have the goal of achieving deep heating of tissue withoutoverly heating tissue directly surrounding the catheter as isparticularly useful for treating benign prostatic hyperplasia withoutdamaging the urethra. A small, highly efficient antenna deliversmicrowave power radiation within a field of the necessary size andvolume to necrose tissue. The catheter also contains a cooling means tocontrol surface temperature and shallow depth heating. This applicationincorporates herein by reference U.S. Pat. No. 5,904,709, filed Apr. 17,1996 and issued May 18, 1999, U.S. patent application Ser. Nos.09/162,457 and 09/154,622 each filed Sep. 16, 1998, and U.S. applicationSer. No. 09/154,989 filed Sep. 17, 1998.

Referring now to the drawings, and more particularly to FIG. 1, thepresent invention discloses catheter 10 with microwave antenna 12 inaccord with the present invention. Although variations for a microwaveantenna to be used with catheter 10 are possible, microwave antenna 12is a presently preferred embodiment of the invention. Microwave antenna12 includes body 14 made of insulating material such as TEFLON.Microwave antenna 12 may be made from a standard coaxial cable having acenter conductor 16 and sheath 20. Because of the small diameter,microwave antenna 12 is especially useful for a catheter. The diameterof microwave antenna 12 may be in the range of about 2 millimeters.Radiation from antenna 12 is directed substantially orthogonally toantenna center conductor 16 or more specifically monopole 26 and isconcentrated between end 18 and feed point 28 which is located at theend of sheath 20. In a homogenous environment, microwave antenna 12 isnon-directional and radiates with radial symmetry. The double-diskmonopole design includes tip disk 22 and tuning disk 24 mounted to pole26 which is an extension of center conductor 16. This design has a verybroad bandwidth so that it can be used over a wide range of frequencieswith little degradation in performance. Therefore, manufacturing andoperating environment variations are well tolerated.

Radiation from antenna 12 occurs mainly from the discontinuities such asfeed point 28, tip disk 22, and tune disk 24. Control of variousfeatures of the antenna may include adjustment of the axial spacings ofthe elements such as the axial spacing between feed point 28 and tip 18,between feed point 28 and tuning disk 24, and between tuning disk 24 andtip disk 22. Such adjustments alter the radiation pattern from antenna12. Other changes that affect the radiation pattern include the diameterof the tuning disk 24 and tip disk 22. Control of these variables andothers discussed in the parent to this application referenced above maybe used to vary the radiation pattern and the resulting heating profile.Computer simulations discussed in the parent to this application andhere subsequently allow manipulation of such variables to obtain theheating profile most similar to that desired.

For use in treatment of benign prostatic hyperplasia without damagingthe urethra, it important to cool catheter 10 in order to save tissue,such as the urethra, immediately adjacent to catheter 10. If microwaveheating is administered via the urethra, the present invention may beused to save the urethra while still thermally necrosing or ablating theenlarged prostatic tissue surrounding the urethra. Fusion material 30disposed around antenna 12 is used to keep catheter 10 cool. Becausecatheter 10 is necessarily in immediate contact with the urethra, theurethral tissue will also remain cool. Fusion material 30 uses the heatof fusion to maintain the temperature at the melting point of fusionmaterial 30. The energy required to change a solid to a liquid whileremaining at the melting point temperature is well known as the heat offusion of a material. When fusion material 30 is heated, its temperaturerises until reaching the melting point. At the melting point, thetemperature remains constant until all of the fusion material hasmelted. Depending on the type of fusion material used, the urethraltissue may remain substantially at body temperature assuming the meltingpoint of fusion material 30 is at or near body temperature. To increasethe volume of fusion material involved, it is generally desirable tohave a compartment of fusion material in catheter 10 not onlysurrounding antenna 12 but also forward of antenna 12 in region 32 andbehind antenna 12 in region 34. Fusion material is held in position bysheath 36 although conceivably if fusion material 30 is completelyharmless to the body, and if desired, sheath 36 could be left off.Another use of sheath 36 is to maintain a certain size catheter suchthat attenuator 38, which is a tubular conductor, may be used to reducemicrowave current flowing back along the outside surface of catheter 10.Attenuator 38 is preferably positioned such that it is at or near thesurface of catheter 10. In the present embodiment, attenuator 38 has asurface 40 positioned just below surface 42 of catheter 10. Attenuator38 is preferably electrically connected to sheath 20.

When catheter 10 transmits microwaves in the urethra and the temperatureof fusion material 30 reaches the melting point, then fusion material 30begins to change from one physical state, the solid state, to a secondphysical state, the liquid state. While changing states or melting,fusion material 30 removes heat from surrounding tissue due to microwaveradiation. This process continues until all of fusion material 30 haschanged to the liquid state. Materials exist that are highly effectivefor cooling purposes due to having a high heat of fusion. More heat canbe removed if the material selected for fusion material 30 has a highheat of fusion. This makes possible a longer heating time, if desired.As examples only, two substances that have desirable properties for usein the present application include dibasic sodium phosphate andphosphonium chloride. Other materials could also conceivably be used.

Fusion material 30 should preferably have a melting point at about bodytemperature or below. A range of presently preferred melting points isfrom about 90° F. to 100° F. In some cases, to effect cooling over agreater radius from catheter 10, it might be desirable to use fusionmaterial with lower melting points. A fairly wide range of values forthe melting point well outside of the above range may be used dependingon the volume of tissue to be protected or, in other words, depending onthe desired temperature profile for tissue heating. However, in thecomputer simulations discussed subsequently, it is assumed that themelting point is at body temperature.

In addition, fusion material 30 should be sufficiently flexible that itwill easily track the path of the urethra. It may be desirable toprovide fusion material 30 in a crystalline, granular, or powder form toprovide sufficient flexibility while fusion material 30 is in the solidstate.

Another criteria for fusion material 30 is that it is microwavetransparent or nearly so. Generally powders and crystals as may be usedfor flexibility also tend to absorb little or no microwave energy so asto be microwave transparent. This feature is desirable so that only theheat energy convexly flowing from the urethra is used to melt fusionmaterial 30 rather than the microwave energy directly received fromantenna 12.

If it is desired to re-use catheter 10, then there is an additionalrequirement that when the material is resolidified, that it is possibleto insure that fusion material 30 will return to the powder or granularform so that catheter 10 remains flexible. It may be necessary toultrasonically powderize the resolidified material or otherwise act onit to effect this purpose. However, due to the simplicity ofconstruction of catheter 10 and the very low cost if produced inquantity, catheter 10 may simply be discarded after use.

Due to the simple construction, catheter 10 may be inexpensivelyproduced with different antenna spacings for different widths and depthsof regions to be heated. The diameter of catheter 10 may vary between avery approximate range of from about 3 millimeters to about 7millimeters depending on urethra size and blockage. The diameter mayalso be considered in determining the desired heating time. The volumeof the catheter increases with the square of the radius so that even aslight increase in radius of catheter 10 significantly increasespotential cooling power.

In some cases, it may be desirable to apply microwave heating via thecolon. The heat of fusion type of cooling discussed above is alsoeffective to protect the colon. Generally, significantly more fusionmaterial 30 can be used when applying heat through the colon due to thepossibility of using a larger catheter. Thus, heating through the colonprovides the possibility of a much longer heating time if that isdesired. As well, when heating through the colon, a modified versioncatheter 10A is preferably used as indicated in FIG. 7 so that antenna12 is effectively directional and colon tissue is not unnecessarilyheated.

A preferred technique for making antenna 12 directional is to includemicrowave absorbing material 44 within the enlarged catheter 10A asindicated in FIG. 7. Microwave absorbing material 44 is not transparentto microwave radiation but instead readily absorbs microwave radiationto substantially limit transmission in undesired directions such as anydirection except toward the prostrate tissue. This technique tends to bemuch simpler than using directional elements in the antenna, althoughdirectional elements could also be used. The substantially increasedamount of fusion material 30 in catheter 10A will offset the heatgenerated within absorbing material 44. Also, since there is somedistance to the prostate region to be heated, the significant radiationpoints 22, 24, and 28 may be moved further apart and phased to providefocusing at some selected depth.

It may also be desirable in certain circumstances to use a thin strip ofabsorbing material, such as absorbing material 44, in catheter 10 foruse in the urethra to thereby reduce radiation in a particulardirection, e.g., such as in the particular direction of the ejaculatoryduct and the like that may be positioned outside the region ofprotection provided by fusion material 30. In this manner, catheter 10for use in the urethra may be made directional if desired.

Thus, there are many possibilities for control of the temperatureprofile to be produced. The relatively simple construction of catheter10 or 10A and the materials are of relatively very low cost as comparedto other catheters, so that numerous variations may be made and perhapsindividually designed catheters may be used without significantlyincreased capital investment. It will be understood that operating andpower control equipment 46 may be used at the end of catheter 10 forcontrolling radiating power for catheter 10.

FIG. 2 shows several temperature versus depth computer projections foruse of catheter 10. For purpose of the simulation, it is presumed thatthe melting temperature of fusion material 30 is body temperature or 38°C. As can be seen in Example 1, which is also displayed threedimensionally in FIG. 3, the temperature in tissue surrounding catheter10 is heated such that in the range of from two to nine millimeters fromthe catheter significant necrosing of tissue will occur. An advantage isthat the heating time is only two minutes so that the treatment isfairly quick. The power level is 3 watts. The volume of tissue heatedover ten degrees centigrade is 807 cubic millimeters. Significantamounts of tissue may be necrosed when tissue temperature is raised byabout ten degrees. The likelihood of tissue ablation is related largelyto the maximum temperature and the time duration of heating. The totalamount of energy deposited or the amount of energy deposited in anylayer can be displayed as desired. Note that the urethra temperaturewould remain cool, but heating beyond the urethra is adequate to necrosecells out to a distance of approximately one centimeter. If deeperheating is needed, this can be obtained with longer delivery timesand/or adjusting focusing, power level, and frequency of antenna 12.However, the delivery time is limited by the total fusion time of fusionmaterial 30. When used though the colon, the heating time can besignificantly increased due to having a greater amount of fusionmaterial as discussed hereinbefore.

In Example 2, the frequency and the power level is the same as that ofExample 1 but the heating time is three minutes. The volume of tissueablated or necrosed will be increased as indicated by the highertemperatures but also by the increased time duration. The volume oftissue heated over ten degrees centigrade is 1295 cubic centimeters.

Example 3 shows the effect of changing the frequency to 450 kHz therebyresulting in another temperature profile. In Example 3, the power levelis 3 watts and the heating time is two minutes. The volume of tissuewith a temperature increase over ten degrees is 811 cubic millimeters.In Example 1 and 2, the conductivity is 0.95 mhos/meter and in Example3, the conductivity is 0.83 mhos/meter.

A different comparison can be obtained by reviewing the isothermallayers of temperature rather than a temperature change in a particularstraight line out from catheter 10 as per the chart of FIG. 2. Onemethod of reviewing the temperature is to provide several slices of aselected cube of simulated tissue surrounding catheter 10. In FIG. 3through FIG. 5, cube 50 of tissue is one of four cubes surroundingcatheter 10. For each variation of transmission characteristics, cube 50is shown in each figure along with eight respective x-y cross-sectionsor slices starting from front of cube 50 and going towards the back ofcube 50 as indicated by the view. Therefore, slice 52 is the front sliceand the next seven slices 54, 56, 58, 60, 62, 64, and 66 showprogressively deeper slices. The antenna is positioned such thatdiscontinuities from which most radiation is broadcast are along the topedge of the selected portion of cube 50 with three discontinuities 68,70, and 72 indicated by the circles. These discontinuities may berelated to tip disk 22, tuning disk 24 and antenna feedpoint 28.

The isothermal zones of temperature are indicated by the respectivetypes of shading although preferably a color diagram would be presented.For this example, the outermost thermal shading zone 74 representstissue that is heated such that the temperature change is less than fivedegrees centigrade. Thermal shading zone 76 represents tissue that isheated such that the temperature change is more than five degreescentigrade but less than ten degrees centigrade. Thermal shading zone 78represents tissue that is heated such that the temperature change ismore than ten degrees centigrade but less than fifteen degreescentigrade. Thermal shading zone 80 represents tissue that is heatedsuch that the temperature change is more than fifteen degrees centigradebut less than twenty degrees centigrade. Thermal shading zone 82represents tissue that is heated such that the temperature change ismore than twenty degrees centigrade.

The exact temperature at which tissue is ablated or necrosed will varydepending on various factors. The length of time the tissue remains atan elevated temperature and the maximum temperature are importantfactors. For relatively short periods of time, tissue heated abovetwenty degrees centigrade is highly likely to be necrosed. In FIG. 4, itcan be seen that a significant or high percentage of the volume oftissue in thermal shading zone 82 will be necrosed due to highertemperature and also the longer heating time. Assuming the catheter ispositioned at the location at which blockage of the urethra occurs, thenit is anticipated that as the tissue dies and is reabsorbed by the bodythen relief may be obtained from the benign prostatic hyperplasiacondition. Tissue in thermal shading zones 80 will also likely benecrosed. As well it is anticipated that at least a significant amountof tissue in shading thermal zone 78 will be necrosed. The effect offrequency and time changes can be seen by reviewing FIG. 3 through FIG.5 where other factors such as cooling adjacent the catheter, powerlevel, tissue characteristics, and antenna characteristics are keptconstant. Further details concerning the various characteristics arediscussed below and in the parent to this application.

Moreover, the tissue immediately surrounding the catheter out to aboutone millimeter is virtually unaffected by the heating. Thus, it isanticipated that a treatment would be relatively quick with fewer sideeffects than other microwave treatments presently used. It will beappreciated from the above that the temperature profile can becontrolled to a great extent and so can made to match a desired pattern.Moreover, the expected response can be tested for numerous variationsusing the computer simulation of the present invention.

FIG. 6 shows the general design of simulation elements for a microwaveradiator system, such as computer simulated system 100. The computersimulation has been written to determine the temperature profiles thatcan be provided in the prostate tissue. The simulation is performed byan accordingly programmed computer in which the program may be storedwithin a storage medium such as a hard disk or diskette. The computereffectively acts as a simulator in accord with the programming that maybe contained in a memory. The inputs to the program include antennacharacteristics, tissue characteristics, the frequency, power level, anddelivery time of the microwave energy. A temperature profile may beproduced as discussed above that shows temperature versus distanceradially or orthogonally outwardly from the catheter or antenna axis.Temperature variations may also be displayed over a selected period oftime.

Simulated catheter antenna 110 generates microwave radiation thattravels through various medium. Although the simulation of the presentinvention may be used for simulating microwave energy radiation intoprostate tissue, it will be apparent that other uses are also available.The antenna will have various characteristics some of which arediscussed in more detail in the parent to this application. In thisembodiment, simulated fusion material 114 preferably forms an outerlayer of catheter antenna 110 and represents the first medium throughwhich the microwave energy must travel. As discussed above, simulatedfusion material layer 114 is preferably transparent to microwaveradiation. To maintain urethral cooling, fusion material 114 is indirect contact with tissue 116. For the outputs shown in FIG. 2 throughFIG. 5, it was assumed that fusion material 114 had a melting point atbody temperature. Microwave energy 112 emerges from catheter antenna 110and travels unopposed through simulated fusion material 114 to engagesimulated tissue 116. For a urethral catheter, fusion material 114 ofcatheter antenna 110 will necessarily be in held in direct contact withtissue 116 due to the relative size of catheter 10 and the urethra.

In the simulation, the microwave energy travels through tissue 116 andheats selected volume elements of simulated tissue referred to ascomputation cells 118. Each computation cell 118 is arbitrarily selectedto be one or two cubic millimeters in size in the presently preferredembodiment of the simulation although this size may be varied. Theenergy applied to these cells by microwave radiation is determined foreach selected time increment. As well, the computer computes the energythat leaves/arrives each computation cell 118 due to thermal conductionfor each computation cell 118 for each selected time increment. In thismanner, a computer simulation can determine the temperature profile forthe tissue over a total desired heating duration. The total desiredheating duration will typically consist of a plurality of short timeincrements.

The inputs to the simulation include, for instance, the conductivity andrelative permittivity of the prostate tissues at higher frequencies.Conductivity is especially important since the conductivity primarilydetermines the rate of absorption of the microwave energy into thetissue and the maximum propagation distance through the tissue.

In a presently preferred embodiment of the simulation, a computationaltissue cube having a size that correlates to a region of tissue to beablated is given the electrical and thermal characteristics of in-vivoprostatic tissue. The cube is subdivided into 8000 small cubes with eachcube being a computational cell such as computation cell 118. Theinstantaneous heat of one arbitrary computational cell in the cube isgiven by:

Q _(C) =Q _(C)°+(ΔQ _(RF) +∫ΔQ _(HC))Δt

where:

Q is the new heat energy in the computational cell;

Q_(C)° is the previous heat energy level;

ΔQ_(RF) is the heat added due to absorption of microwave energy;

∫ΔQ_(HC) is the net heat added or lost by the cell from heat conduction;and

Δt is a small time constant.

The new temperature of the cell is given by:

T _(C) =Q _(C) /MS

where:

T_(C) is the new cell temperature in °C.;

M is the mass of the cell; and

S is the specific heat of the cell.

Each cell is assumed to be a cube with six faces. The heat energytransferred through each face for one time increment is given by:

ΔQ=−KA(∂T/∂r)Δt

where:

ΔQ is the heat transferred through one face;

K is the thermal conductivity of the cell;

∂T/∂r is the temperature gradient from the center of one cube to thenext; and

A is the area of one face.

The electric field intensity in a cell is given by:${\hat{E}}_{1} = \frac{{\hat{E}}_{01}^{{- \gamma}\quad r_{1}}}{r_{1}^{2}}$

where:

Ê₁ is the electric field intensity resulting from the radiation at thefeed point of the antenna;

Ê₀₁ is related to the relative magnitude and phase of radiation from thefeed point;

γ=α+jβ;

α is the attenuation constant associated with the tissue;

β is the phase shift constant; and

r₁ is the distance from the antenna feed point to the center of a cell.

The total electric field at a cell due to radiation from the feed point,middle disk and top disk (each being a microwave radiator) is given by:

Ê=Ê ₁ +Ê ₂ +Ê ₃

where:

Ê₂ is calculated similarly to Ê₁ except using r₂ to the mid-disk; and

Ê₃ similarly using top disk associated r₃.

Finally, the energy absorption at the cell is given by:

 W _(a) =vσ|E| ² Δt

where:

W_(a) is the electromagnetic energy absorbed;

v is the volume of the cube; and

σ is the conductivity of the medium.

The results from the simulation can be plotted in numerous ways such asthose disclosed above. As well, given a particular desired profile thenecessary input characteristics can be obtained such as power levels,operating time and frequency. Thus, the simulation can be used todetermine results from particular inputs or to calculate the necessaryinputs to obtain desired results.

In operation of the present invention, a computer simulation may be usedto determine to predict what the temperature profile will be in theprostatic tissue. The temperature profile can be modified until itappears to fit the desired result. The amount of tissue to be necrosedcan be calculated. The procedure goes fairly quickly. The heat of fusionmaterial 30 should be in a solid state although flexible. The catheteris then positioned in the urethra at the desired position at whichprostatic tissue is to be selectively removed. The microwave antenna isturned on at the predetermined power level for the predetermined heatingtime. The urethra is undamaged due to the heat of fusion material 30. Inthe next several weeks, the body reabsorbs necrosed tissue therebyproviding relief to the patient of benign prostatic hyperplasiasymptoms.

While the preferred embodiment of the catheter apparatus and methods aredisclosed in accord with the law requiring disclosure of the presentlypreferred embodiment of the invention, other embodiments of thedisclosed concepts may also be used. Therefore, the foregoing disclosureand description of the invention are illustrative and explanatorythereof, and various changes in the method steps and also the details ofthe apparatus may be made within the scope of the appended claimswithout departing from the spirit of the invention.

What is claimed is:
 1. A method of constructing a transcathetermicrowave antenna, comprising: providing a cable with one or moreconductors to form a catheter operable for insertion into the body;adapting said cable for connection to a microwave power source;providing a microwave antenna at one end of said cable; providing alayer of microwave transparent fusion material in a first physical stateadjacent said microwave antenna, said fusion material being alterable toa second physical state from said first physical state to provide heatof fusion cooling adjacent said catheter during operation of saidmicrowave antenna.
 2. The method of claim 1, further comprising:selecting said microwave transparent fusion material to have a meltingtemperature in the range from about eighty to one hundred degreesFahrenheit.
 3. The method of claim 1, further comprising: providing thatsaid first physical state is a solid physical state, and selecting saidmicrowave transparent fusion material in said solid physical state to besufficiently flexible for use with said catheter such that said catheteris bendable within the body.
 4. The method of claim 1, furthercomprising: providing that said microwave transparent fusion material isin surrounding relationship to said microwave antenna.
 5. The method ofclaim 1, further comprising: providing that said microwave antenna isdirectional.
 6. The method of claim 1, further comprising: providingthat said fusion material is a powdered substance when in said firstphysical state.
 7. The method of claim 1, further comprising: providingthat said fusion material is a crystalline substance when in said firstphysical state.